Ovulation is the process where an ovum or ova are released from the ovaries. The timing of ovulation within the menstrual cycle is of foremost importance for fertilization. Since the life span of both spermatozoa and the unfertilized ovum is limited, fertilization must take place within hours after ovulation if conception is to occur during that menstrual cycle.
Ovulation is under the control of circulating estrogen and progesterone levels from the ovary and gonadotropins from the pituitary. During the normal menstrual cycle in women these hormones exhibit cyclic patterns. The menstrual cycle can be functionally divided into three phases; the follicular, the ovulatory and luteal phases. The follicular period begins in the late luteal phase of the preceding cycle with a rise in blood levels of follicle stimulating hormone (FSH, a gonadotropin) and a concomitant initiation of ovarian follicular growth. Luteinizing hormone (LH, the other gonadotropin) blood levels also rise but start one or two days later than FSH levels. In the second half of the follicular stage, ovarian secretion of estradiol (E.sub.2) and estrone (E.sub.1) by the ovary increases slowly at first, then rapidly reaches a maximum on the day before the LH peak. The rise in plasma estrogen levels is accompanied by a decrease in FSH levels.
During the ovulatory phase there is a rapid rise in blood LH levels which leads to the final maturation of the ovarian Graafian follicle, follicular rupture and discharge of the ovum some 16 to 24 hours after the LH peak. Just prior to ovulation blood E.sub.2 levels drop dramatically and plasma progesterone levels begin to rise.
Following ovulation, during the luteal phase, the post-ovulatory ovarian follicle cells are luteinized to form a corpus luteum. The most important feature of the luteal phase of the menstrual cycle is the marked increase in progesterone secretion by the corpus luteum. There is a smaller increase in estrogen levels. As progesterone and estrogens increase, LH and FSH decline throughout most of the luteal phase but FSH begins to rise at the end to initiate follicular growth for the next cycle.
Progesterone and estrogen secretion by the ovary are controlled respectively by levels of LH and FSH. Negative and positive feedback inhibition of progesterone and estrogens regulate the hypothalamus to control luteinizing hormone--releasing hormone (LHRH also termed gonadotropin releasing hormone, GnRH). During periods of high circulating blood levels of progesterone and estrogen, low amounts of LH-RH are produced. Inversely, when progesterone and estrogen levels are low in the blood, high amounts of LH-RH will be produced. However, some progesterone is required to initiate the LH surge. LHRH synthesis in the hypothalamus stimulates the anterior pituitary to synthesize and secrete LH and FSH.
A side view of the brain with hypothalamus and pituitary enlarged is shown in FIG. 1. Also shown is the ovary in the follicular, ovulatory and luteal phase which produce estrogen and progesterone in the follicular cells in response to stimulation respectively from FSH and LH. High levels of progesterone and estrogen feed back on the hypothalamus and negatively regulate the secretion of LH-RH or GnRH and to decrease production of LH and FSH. During periods of low serum levels of estrogen and progesterone LH-RH levels rise to stimulate synthesis of FSH and LH. However, both estrogen and progesterone also have positive feedback control on the hypothalamus and some progesterone is required for stimulating LH-RH. It is on the basis of this concept that the modern contraceptive "pill" is designed. Progestins and estrogens in the "pill" inhibit the synthesis of LH-RH thus preventing the LH surge which is required for stimulation of growth, maturation and rupture of the Graafian follicle.
Female contraception methods are based upon the above theory of the control of ovulation. Generally, all contraceptive procedures are based upon the principal that high or moderate progesterone or estrogen levels inhibit LHRH and the LH surge and thus prevent ovulation. Thus, estrogen and/or progesterone are typically prescribed to inhibit ovulation. In the USA alone, about 75 million women take birth control pills to control ovulation and prevent pregnancy. The methods of hormonal regulation of fertility can be outlined as follows:
1. Oral contraception. PA1 2. Long acting injectable hormone preparations. PA1 3. Hormone--releasing intrauterine systems. PA1 4. Interception--usually a large dose of estrogen in cases of unprotected intercourse. PA1 5. Antiprogesterones--which block action of progesterones. PA1 6. LHRH antagonists or agonists both of which interfere with normal processes and inhibit steps in ovulation. PA1 7. Antigonadotropins--such as Danazol which is thought to block implantation.
a) Cyclic combined estrogen--progesterone method. PA2 b) Sequential estrogen and progestogen method. PA2 c) Continuous (noncyclic) low-dose, progesterone only treatment.
Potential users of these hormone contraceptives should be alerted to the fact that both hormone components may be associated with a slightly increased risk of cardiovascular disease. In an asymptomatic woman younger than 35 years, the risk is not a deterrent to use but should be considered additive to other cardiac risk factors. Thus, hypercholesterolemia, hypertension, diabetes, heavy smoking, or a family history of early coronary disease may augment the risk. Discontinuance of oral contraceptives and use of an effective alternative should be considered in the management of hypertension or major glucose intolerance. Use of these agents by women older than 35 years should be avoided by those who smoke and reevaluated for others.
Absolute contraindications to oral contraceptives include thrombotic disorders, known or suspected cancer of an estrogen-dependent organ (e.g., breast or uterus), impaired liver function, pregnancy, undiagnosed vaginal bleeding, pregnancy-associated jaundice, and hyperlipidemia. In many other disorders, a relative contraindication should be individually evaluated and use of oral contraceptives cautiously explored. Because the frequency of arterial thrombosis appears to be increased after elective surgery, it is recommended that oral contraceptives be discontinued a month before surgery.
The present invention offers many advantages over the normal hormonal regulation of ovulation because the methods and compositions use either no estrogen and progesterone or lower amounts of these hormones than current methods.
Agents which stimulate ovulation also function by acting on the above pathways. The best known agent which stimulates ovulation and is used for treatment of anovulation is clomiphene (MER 41). Clomiphene is a nonsteroidal antiestrogen that competes for estrogens at their binding sites. It is thought that clomiphene binds to estrogen receptors in the hypothalamus and blocks the negative feedback exerted by ovarian estrogens. The result is increased output of gonadotropins and stimulated follicle growth and maturation.
The present invention relates to the interaction of the above mechanisms and hormones with the production of nitric oxide. Nitric oxide was originally shown to be produced by the endothelium of blood vessels and to regulate vascular tone/blood pressure (Moncada, et al. 1991). However, nitric oxide has been shown to be synthesized by many tissues including the central and peripheral nervous systems (Snyder and Bredt, 1991) and the uterus (present inventors). European patent application EP0441119A2, incorporated by reference herein, appears to disclose the use of L-arginine (the donor substrate for nitric oxide) in the treatment of hypertension and other vascular disorders. The publication suggests that the mechanism by which L-arginine is effective for this purpose is because it may be "the most powerful endothelial-derived releasing factor, nitric oxide." U.S. Pat. No. 5,028,627, incorporated by reference herein, appears to disclose the use of certain arginine derivatives to inhibit nitric oxide production from arginine in the treatment of systemic hypotension.